Medical equipment for radiation therapy treats tumorous tissue with high energy radiation. The dose and the placement of the dose must be accurately controlled to insure both that the tumor receives sufficient radiation to be destroyed, and that damage to the surrounding and adjacent non-tumorous tissue, referred to as organs at risk (OAR), is minimized.
Radiation therapy typically uses a radiation source that is external to the patient, typically either a radioisotope, such as cobalt-60, or a high energy x-ray source, such as a linear accelerator. The external source produces a collimated beam directed into the patient to the tumor site. However, external-source radiation therapy undesirably irradiates a significant volume of OAR in the path of the radiation beam along with the tumorous tissue. The adverse effect of irradiation of healthy tissue may be reduced, while maintaining a given dose of radiation in the tumorous tissue, by projecting the external radiation beam into the patient at a variety of “gantry” angles with the beams converging on the tumor site. The particular volume elements of healthy tissue, along the path of the radiation beam, change, reducing the total dose to each such element of healthy tissue during the entire treatment.
The irradiation of healthy tissue also may be reduced by tightly collimating the radiation beam to the general cross section of the tumor taken perpendicular to the axis of the radiation beam. Numerous systems exist for producing such a circumferential collimation, such as systems with multileaf collimators. The multileaf collimator (MLC) may control the width and offset of the radiation beam as a function of gantry angle so that tumorous tissue may be accurately targeted.
Collimation is just one way of modulating the radiation beam. Additionally or alternatively, the radiation beam may be attenuated. Collimators control the outline of the radiation beam; attenuators control the intensity of the radiation beams that are beamed at the tissue. Phrased more technically, collimators block radiation so as to create a generally binary spatial intensity distribution, while attenuators typically produce continuous spatial modulation of the beam intensity by selective attenuation.
For example, Intensity Modulated Radiotherapy (IMRT) is aimed at irradiating a target while protecting healthy tissue, especially organs-at-risk (OAR). Intensity modulation is implemented either by multileaf collimators or by attenuating modulators. A desired intensity map is approximated by segmentation: forming a sequence of aperture segments consecutively shaped by an MLC.
The apertures (and associated respective intensities) may be modified continuously during irradiation, producing what is generally called Dynamic IMRT. Dynamic modulation is possible by continuous irradiation while changing the orientations and/or the apertures together with the respectively associated intensities.
A single arc IMRT has been described. The procedure involves rotating the beam about the target for an arc of one or two revolutions, while MLC-apertures and associated intensities are continuously modified. Although the rotational speed is low—about one revolution per minute—modulation performance is limited by the short time-interval allocated for each orientation increment.
A theoretical approach applicable to a single arc IMRT has been described by Brahme et al: “Solution of an integral equation encountered in rotation therapy”, Phys. Med. Biol., 27, (1982), No. 10, 1211-1229. An analytical expression is presented for the parallel beam profile in each orientation for obtaining uniform target dose while protecting a central organ, whereas the target and the organ are concentric circles. Extension of the expression to non-concentric circles was observed by Bortfeld et al: “Single-Arc IMRT?”, Phys. Med. Biol., 54, (2009) N9-N20. Beam modulation may be implemented by a “sliding window” MLC technique whereby continuous exposure time is spatially controlled.
IMRT can also be implemented by compensators, also referred to as attenuating modulators. A compensator uses a two-dimensional attenuating pattern that modulates the beam intensity by spatially-selective attenuation. An example of this is described in “Compensators: An alternative IMRT delivery technique”, Sha X. Chang et al, Journal of applied Clinical Medical Physics, Vol 5, No. 3 (2004).
IMRT via 2D attenuating modulators obviates segmentation. Attenuating modulators are fabricated respectively for each of the 5-7 orientations selected for the treatment. A respective attenuating modulator is placed in position prior to each oriented irradiation. A single-arc IMRT using 2D attenuating modulators would be prohibitively expensive, since a large set of modulators would have to be fabricated for each patient and it is complicated to replace/move them at high speed.